A Visitor from Another Star System

China's Tianwen-1 spacecraft has photographed the interstellar object 3I/ATLAS as it passed near Mars, the China National Space Agency (CNSA) announced. The orbiter used its high-resolution camera to capture the comet from about 30 million kilometers (18.6 million miles) away.

Tianwen-1 has been orbiting Mars for four years and eight months, making it one of the closest spacecraft to observe the interstellar object since its discovery on May 7, 2025.

Tail and Coma Revealed

The new images show several features confirming 3I/ATLAS is a comet. A visible tail extends behind the object, while a cloud of gas and dust — called a coma — surrounds its nucleus. Researchers combined multiple images into an animation that traces the comet's movement as it travels toward its closest approach to the Sun.

The observations add to growing evidence that 3I/ATLAS contains water and other volatile materials that began escaping as the object warmed during its journey through the inner solar system.

A Difficult Target to Capture

Photographing 3I/ATLAS was extremely challenging. The comet is both distant and relatively small at 5.6 km (3.5 mi) in diameter. Its speed added another major challenge — the comet was moving at approximately 58 km/s (36 mi/s) relative to the Sun, while its motion relative to Tianwen-1 reached roughly 86 km/s (53.5 mi/s).

The High-Resolution Imaging Camera (HiRIC) aboard Tianwen-1 was not originally designed to observe such dim objects — it was built primarily to study bright features on the Martian surface. The team responsible for the camera began preparing for the encounter in early September, conducting repeated simulations, theoretical modeling, and detailed instrument inspections.

A Coordinated Interplanetary Observation

On October 3, both European orbiters also photographed 3I/ATLAS. Mars Express used its High Resolution Stereo Camera (HRSC), while the ExoMars spacecraft relied on its Colour and Stereo Surface Imaging System (CaSSIS). The comet was exceptionally difficult to detect — when compared with sunlit features on Mars, 3I/ATLAS appeared roughly 10,000 to 100,000 times fainter.

What Interstellar Objects Tell Us

Like the two interstellar objects identified before it — 'Oumuamua and 2I/Borisov — 3I/ATLAS offers unusual opportunities for astronomy and astrobiology. Asteroids and comets preserve leftover material from the formation of planetary systems. An object arriving from interstellar space may contain clues about conditions around another star.

Studying such a visitor could reveal how other planetary systems formed and what kinds of materials they contain. A dedicated mission designed to intercept interstellar objects could gather information that scientists cannot obtain through ordinary telescopic observations.

Future Interceptor Missions

Scientists hope that future spacecraft will be prepared to intercept newly discovered interstellar objects at close range. One effort already under development is the European Space Agency's Comet Interceptor mission, expected to be completed by 2029. It will wait in space for an opportunity to investigate a suitable comet or other promising target.

The successful observation also provides a preview of the type of work China hopes to conduct with Tianwen-2, a mission expected to visit a Near Earth Asteroid and later study a comet.

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